The present invention relates to apparatus for the clarification of water where the removal of suspended particles is accomplished by flotation using a stream of rising, microscopic air bubbles, and usually a flocculation agent. More specifically, it relates to an improved clarification apparatus characterized by a shallow, cylindrical flotation tank and a rotating scoop for removal of floated sludge of the type described in U.S. Pat. Nos. 4,022,696; 4,377,485; and 4,626,345.
Water clarification, the removal of particulate contaminants suspended in water, is used to treat waste water from manufacturing processes, particularly in the paper and pulp industries, in the treatment of municipal water drinking supplies, and in sewage treatment. The water can be clarified by sedimentation or flotation of the particles. Known sedimentation techniques and apparatus are efficient, but are limited even at peak efficiency to a comparatively slow sedimentation rate, about 0.5 gallons per minute per square foot. To handle large volumes of raw input water, sedimentation facilities must therefore be large, with attendant cost and space utilization disadvantages.
Flotation techniques dissolve a few percent of air by volume in pressurized water and then release the air in the form of microscopic bubbles which attach to the particles and carry them upwardly to the surface where they form a floating sludge. The particles are usually flocculated using conventional flocculating agents such as alum before the air bubbles are introduced. Flotation techniques are theoretically capable of achieving clarification rates of 7.5 gallons per minute per square foot of flotation area. Heretofore in practice the rates have been less than this theoretical value, but significantly better than for sedimentation techniques.
Known apparatus to dissolve air in pressurized water use a single tube which is valved to admit water which is pumped to a high pressure, a typical value being 6 atmospheres. Since water is substantially incompressible, it requires a large power input to a centrifugal pump to produce this pressure level. Compressed air is added to the pressurized water, preferably with the water swirling through a zone in the tube where the compressed air is introduced in order to enhance the dissolution of the air into the water. After a sufficient mixing, water reaches an outlet where a valve discharges to a low pressure pipe, typically at 2 atmospheres, which directs the pressurized, aerated water to a discharge manifold, or the like, in the flotation tank.
Applicant holds several U.S. patents for water clarification apparatus and processes. Early attempts described in U.S. Pat. Nos. 2,874,842 issued in 1959 and 3,182,799 issued in 1965 used a stationary tank with no skimmers or other moving components in the tank. The gas bubbles were introduced via the main raw water inlet and guided within the tank by an internal deflector ('842) or a stack of internal baffles ('799). Because the gas bubbles were guided by stationary components, there was no design problem created by the turbulence of moving parts in the flotation tank. Also these devices did not lend themselves to treatment at high flow rates. The '799 apparatus had the additional problems in that (i) the inlet water had to be separately fed from the side to the region between each adjacent pair of baffles and (ii) the flow paths for floated particles varies depending on the vertical position of the associated baffles defining the flow path. This latter situation means that the apparatus either does not fully treat the inlet water or is slow.
Significant improvements were presented in applicant's U.S. Pat. Nos. 4,022,696; 4,377,485; 4,626,345; and 4,184,967. In the '696 clarifier, sold under the trade designations "SPC" and "Supracell", the flotation occurs in a circular tank. The raw water is fed into the tank via a central pipe, a hydraulic joint, and an inlet pipe with multiple outlets immersed in the tank which rotates about the tank. The inlet flow is oriented opposite to the direction of rotation of the inlet pipe and is at a velocity with respect to the rotation rate such that the raw water has a net zero velocity as it enters the tank. The raw water inlet flow assembly and a scoop for removing the floated sludge are mounted on a carriage that rotates about the tank. The rate of rotation is set so that the floated particles will reach the surface of the water held in the tank in the time of one rotation. Flocculation occurs in a flocculation chamber, an assembly of walls, pipes and baffles, that is immersed in the flotation tank and rotates around the tank supported by the carriage. Dissolved air is introduced to the flotation tank by a rotating manifold that follows the flocculation chamber. The net zero velocity results in a substantially vertical path for the rising air bubbles and attached flocs. With a flotation rate of approximately 12 inches per minute, a good degree of clarification can be achieved with each rotation of the carriage using a comparatively shallow tank. e.g. 16-18 inches.
In practice the '696 invention has proven to be a highly effective clarification unit, particularly when used with the multi blade rotating scoop of applicant's U.S. Pat. No. 4,184,967. This scoop has a central sludge discharge pipe that receives sludge scooped by the blades. There are openings in the pipe which allow the sludge to enter the center of the pipe. The pipe is inclined so that the sludge flows downhill within the pipe to a collection compartment.
U.S. Pat. No. 4,377,485 discloses a later clarifier of the present applicant sold under the trade designations "SAF" and "Sandfloat", which utilizes the basic approach of the '696 invention, also operating with the net zero velocity principle, but also adding a second filtration stage with a set of wedge shaped sand beds covering the bottom of the flotation tank. The water clarified by the flotation process flows through the underlying sand beds into a clarified water collecting chamber. A suction apparatus mounted on the carriage periodically backwashes each section of the filter. As in the '696 clarifier, a rotating flocculation chamber has vertically aligned, concentric baffles. In the '485 patent, however, the baffles are corrugated so that as the carriage rotates the corrugations gently mix the raw water with the flocculating agent. When the flocs are formed, pressurized, aerated water is introduced to the open, lower end of the flocculation chamber via a manifold with multiple outlets. The bubbles carry the floced particulates upwardly to form a floated sludge layer. The '485 "sandfloat" system provides a two stage clarification, flotation and filtration, which provides an enhanced degree of clarification but with certain attendant increases in cost. The clarification rate, however is comparable to that of the '696 "supracell" clarifier.
U.S. Pat. No. 4,626,345 discloses a clarifier, sold under the trade designations "SASF" and "Sandfloat-Sedifloat". It does not utilize the net zero velocity principle, but does employ a sand bed for two stage clarification. In this apparatus, the raw water enters a central cylindrical compartment, which acts as a hydraulic flocculator, and then flows over a dividing wall into a surrounding circular flotation tank. Aerated water is added via a vised manifold within the flotation tank. Rotating elements include a scoop remover for the floated sludge and a suction hopper that cleans the filter beds. The scoop is the same type as used with the supercell and sandfloat units described above. This '345 SASF apparatus offers a two stage clarification with a more compact unit and at a lower cost than the SAF type clarifier.
In all of these clarifiers, the Supracell, sandfloat and SASF, there are common design features and limitations. For one, the flotation of the floced particles is generally vertical, and is in a body of water that is comparatively shallow and generally free from turbulence. Also, in all of these designs the removal of the floated sludge is by a bladed, rotating scoop feeding an inclined discharge pipe. Further, a principal operating cost is the power required to operate the pumps to pressurize the raw inlet water and dissolve air in it. Also, heretofore, in order to increase the capacity of a given type of clarifier, one built a larger diameter tank. While in theory one can build a large enough tank to accommodate any clarification load, cost and space constraints have provided practical limitations on the capacities of these units.
It is therefore a principal object of this invention to provide an improved water clarification apparatus that can increase the clarification rate of any circular tank flotation type clarifier by a factor of at least two with no attendant increase in the size of the unit.
Another principal object is to provide the foregoing increase in productivity with a comparatively small increase in cost and in a manner which is compatible with existing dissolving apparatus which significantly lowers the cost of operation of the clarifier while efficiently aerating and pressurizing the inlet water.
Yet another object is to provide an improved floated sludge removal apparatus that can remove and transport increased volumes of and more viscous floated sludge loads as compared to known circular tank clarifiers.
A further object is to provide a control system for a clarifier with the foregoing advantage that senses the sludge layer-water boundary and can also sense the turbidity of the water in the flotation tank as a function of depth and in response thereto automatically controls chemical addition and coordinates the operation of the clarifier.